Heat pipe with a sintered capillary structure

ABSTRACT

In a heat pipe, a sintered capillary structure is provided on both of its heat-receiving and heat-delivering surfaces with the sintered capillary structure including both coarse and fine pores. In a preferred arrangement the fine and coarse pores are each distributed in a range of sizes with a maximum range of fine pores of certain sizes in the range of the fine pores and a maximum range of coarse pores of certain sizes in the range of the coarse pores. The coarse pores are formed in the sintering operation while the fine pores can be formed in a number of different ways. In one method the fine pores are formed by using different sizes and shapes of grains in the metal powder which is sintered. In another method, a chemical treatment of the sintered structure provides the fine pores.

[ Oct. 8, 1974 1 HEAT PIPE WITH A SINTERED CAPlLLARY STRUCTURE [75] Inventors: Wilfried Fischer, Neckargemund;

Gregor Gammel, Dossenheim, both of Germany [73] Assignee: Brown, Boveri & Cie AG,

Mannheim, Germany [22] Filed: Apr. 20, 1972 [21] Appl. No.: 245,821

[30] Foreign Application Priority Data UNITED STATES PATENTS 7 1970 Schmidt.. 8/1970 Brown..... 10/1971 Feldman,

3,661,202 5/1972 Moore, .lr. 165/105 Primary Examiner-Albert W. Davis, Jr. Attorney, Agent, or Firm-Toren, McGeady and Stanger- [57] ABSTRACT In a heat pipe, a sintered capillary structure is provided on both of its heat-receiving and heat-delivering surfaces with the sintered capillary structure including both coarse and fine pores. In a preferred arrangement the fine and coarse pores are each distributed in a range of sizes with a maximum range of fine pores of certain sizes in the range of the fine pores and a maximum range of coarse pores of certain sizes in the range of the coarse pores. The coarse pores are formed in the sintering operation while the fine pores can be formed in a number of different ways. lnone method the fine pores are formed by using different sizes and shapes of grains in the metal powder which is sintered. In another method, a chemical treatment of the sintered structure provides the fine pores.

3 Claims, 4 Drawing Figures l HEAT PIPE'WITH A SINTERED CAPILLARY STRUCTURE SUMMARY OF THE INVENTION The present invention is directed to a head tube having a capillary structure formed at least on its heatreceiving and heat-delivering surfaces and, more particularly, it is directed to a sintered capillary structure containing an arrangement of both fine and coarse pores within the capillary structure.

Heat pipes have the characteristic of transferring large quantities of heat with only a small temperature difference occurring in the surfaces between which the heat is transmitted. The temperature difference AT which, though small, is not to be neglected in conven tional heat pipes, and is composed of the temperature drop due to the heat conduction through the heat pipe wall in the heat receiving zone, ATA, the temperature drop within the heat pipe ATa occurring as a result of the heat transfer from the heat pipe wall to the working medium within the heat pipe, the corresponding values of ATA' and ATa' at the heat-delivering surfaces of the heat pipe, and the temperature gradient AT,, which occurs during the transport of the vaporized working medium through the interior of the heat pipe.

While AT is in many cases on the order of one-tenth of a degree and ATA ATrrTAis on the order of several degrees, ATa ATa' often amount to a multiple of 10. Accordingly, to improve the heat conductivity of heat pipes, itis especially important to make the sum of AToz and ATa as small as possible. This temperature difference, which is due to heat transfer to the liquid working medium can be reduced by enlarging the heat transfer area. The enlargement of the heat transfer area is provided in the interior of the heat pipe by means of a capillary structure at the surface where heat transfer takes place so that the ratio of heat transfer area inside the heat pipe to the'corresponding outside area, from which the heat is removed, is greater than one. To a degree the desired effect is obtained in any heat pipe, because the transport of the liquid working medium requires a capillary structure, that is, the inner surface of the heat pipe is increased due to the capillarystructure. ln heat pipes which use grooves r threads in the inner surface of the pipe for effecting the liquid transport or where a cellular structure is used, the increase in the inner surface is slight and, as a result, the reduction of ATa and ATa is at most relatively small. An additional factor in heat pipes using cellular structures is that the heat contact between the lattice or grid forming the cellular structure and the heat pipe wall or between differentsuperposed lattices or grids is generally poor, and the increased inner surface area cannot be fully utilized because the passage of heat to it is accomplished only imperfectly.

Therefore, it is the primary object of the present invention, to provide a heat pipe construction where a substantial increase in the heat transfer area is afforded with a resultant reduction in the temperature difference between the heat-receiving and heat-delivering surfaces, and with the temperature drop within the working medium itself being reduced. Furthermore,

the disadvantages of known capillary structures can be.

avoided.

Another object of the invention is to provide different methods for the production of the capillary structure within the heat tube for achieving the increased heat transfer area.

In accordance with the present invention, the problem experienced in the past is solved by only partially filling the capillary structure with a working liquid.

A sintered capillary structure formed of a metal powderof the same grain size exhibits a pore size distribution curve in which the maximum number of particular sizes covers a Wide range. As a result, there is a certain, if small, number of pores of a smaller diameter. Accordingly, sufficient working medium'is supplied into the tube to fill the smaller pores, while the larger pores remain free of the working medium. It should be noted, however, that if the quantity of working medium is too small, there is the danger of the heat-receiving surface drying out.

To avoid the problem of too small a quantity of the working medium being located at the heat-receiving surface, the capillary structure is formed, at least in the heat-receiving and the heat-delivering surfaces, so that the pore sized distribution curve exhibits two pronounced maximum ranges of pore sizes.

By filling a suitable quantity of the working medium into the heat pipe it can be arranged that the fine pores are filled with the working liquid while the coarse pores do not contain any of the liquid. In this way it is possible to provide a large heat transfer area in the heat pipe surface for the working medium while, at the same time, affording a large transfer area for the liquid-vapor working medium.

The advantage of the capillary structure formed in accordance with the present invention exists in that the temperature drop due to heat transfer in the working medium is greatly reduced. This reduction is accomplished, on the one hand, in that a-large contact area between the heat pipe wall and the liquid working medium is provided and, on the other, a large evaporation surface is provided. In particular, this advantage occurs in the range of ebullient boiling, that is, at the relatively great heat flow density, where the vapor bubbles formed in known capillary structures must first penetrate through the porous structure with a resultant pressure loss. Because of the uniform vapor pressure, a higher temperature corresponds to the higher pressure at which the vapor bubbles are formed, that'is, a temperature drop occurs in the porous layer. This disadvantage existing in known heat pipe constructions is avoided by means of the present invention.

Further, in accordance with the present invention, there are three different methods for producing the desired capillary structure in which the pore size distribution curve contains two maximum ranges of pore sizes.

the formation of the desired fine pores.

In another method the capillary structure can be formed by sintering with a metal powder in which the grains are all of the same material and of the same size with the fine pores being formed by a chemical treatment after the sintering operation, for example, by providing alternating oxidizing and reducing conditions.

Moreover, in the third method the metal powder to be sintered consists of grains of two different sizes with the larger of the grains having an oblong configuration and the smaller ones having a spherical configuration.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there are illustrated and described preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWING In the drawing:

FIG. 1 is a cross sectional view of a portion of a heat pipe illustrating a sintered capillary structure formed thereon in accordance with the present invention;

FIG. 2 is a pore size distribution curveof the pores formed in the sintered capillary structure in FIG. 1;

FIG. 3 is a graph of the heat flow density in two comparable heat pipes based as a function at the AT, with one heat pipe having a conventional capillary structure and the other having a capillary structure formed in accordance with the present invention; and

FIG. 4 is a cross sectional view of a heat pipe with a capillary structure formed in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION In FIG. 1 a sintered capillary structure 2 is shown formed on the inside surface of a heat tube 1. The sintered capillary structure 2 contains large pores 3 and small pores 4. In the sintering operation, two fractions of a metal powder formed of the same material as forms the heat tube, are mixed and sintered quickly on the inner surface of the heat pipe 1. One of the fractions of the metal powder is coarser than the other and consists of particles having a size of about 1 mm. The other fraction is made up of particles having a size of about 50 microns. Preferably, the coarser particles have an oblong configuration while the finer particles have a spherical configuration. File shavings, for example, are suitable for the coarse particles. It has been found, if the coarse particles are spherically shaped, the spaces between them become filled or almost completely filled with the fine particles so that no or only relatively few coarse pores are formed.

In FIG. 2 a pore sized distribution curve is shown, with the pore size indicated along the abscissa and the range or number of pores of a particular size shown along the ordinate. The distribution curve in FIG. 2 is illustrative of the distribution of the pores shown in the capillary structure in FIG. 1.

As can be seen in FIG. 2, two ranges of maximum quantities of pores are indicated by the curve one maximum quantity or range being located in the portion of the curve representing the finer pores and the other maximum range being located in the portion of the curve indicating the coarser pores. When a quantity of liquid working medium is introduced into the heat pipe which corresponds to the free pore volume of the small pores, then, because of the greater capillary force, only the small or fine pores become filled with the liquid while the larger pores remain free of the liquid. In FIG. 2 the liquid within the small pores is indicated by the hatching. To obtain the desired effect, it is not absolutely necessary that two maximum ranges of pore sizes be provided in the pore size distribution curve, as is indicated in FIG. 2, rather it suffices if the pore size distribution curve indicates a single broad maximum of small pores so that only the small pores are filled and not the larger pores.

A similar capillary structure having the same pore size distribution curve is obtained in the following manner.

The capillary structure is formed by sintering a metal powder of two components on the inner surface of the heat tube. The heat particles of the two components can be the same but they need not be. One of the components is a metal powder and the other component consists of an alloy formed of two metals one of which is the same metal as in the metal powder forming the other component. After the sintering operation has been completed, providing the coarse pore arrangement in the capillary structure, the finer or smaller pores can be provided by means of a chemical treatment. For example, if the first component is a nickel powder then the second component is a nickelaluminum alloy, after the sintering operation, the aluminum can be dissolved out of the alloy with potash lye The dissolution of the aluminum forms finer or smaller pores in the capillary structure than the coarse pores formed in the sintering process so that a combination of fine and coarse pores are provided. The capillary structure as shown in FIG. 1, achieves its desired purpose only if care is taken that neither too much nor too little of the liquid working medium is contained in the capillary structure. The proper loading of the capillary structure with the working medium can be achieved in either of two ways. In the optimum case, all of the fine pores are filled with the liquid working medium while all of the coarse pores are left free. The simplest method for accomplishing this loading is to fill the precise amount of working medium into the heat pipe so that the desired effect is achieved. When this is done, the fine pores are filled with the working medium in accordance with that shown in the distribution curve in FIG. 2.

As an alternative to charging the heat pipe with the working medium as indicated above, in a horizontally arranged heat pipe in which heat is supplied into one end and removed from the opposite end, a sufficient amount of working medium is introduced so that the working medium stands in the bottom of the tube a little bit above the pores present in its walls. At the end faces the liquid rises into the fine pores, however, but not into the coarse pores due to the capillary action.

In FIG. 4 a heat pipe having a capillary structure in accordance with the present invention is shown. The excess liquid working medium 5 stands in a pocket or recess below the heat-receiving surface of the heat pipe. The capillary structure 8 lines the entire interior surface of the heat pipe 7. The supply of heat into the heat pipe is illustrated by arrows 9 and the removal of heat from the heat pipe is shown by the arrows 10. In this arrangement, the working medium rises only into the fine pores of the capillary structure at the points where heat is either supplied or removed.

The reason the pore size distribution should correspond to that indicated in FIG. 2, is as follows: theorectically, any pore distribution is suitable for achieving the effect described herein. Even if the pore size distribution has a very limited maximum range, at least by filling with a suitable quantity of the liquid working medium, thefine pores, those pores whose radius or size is only slightly less than that of the coarse pores, become filled while the coarse pores remain open or free of the working medium. However, in practice the filling of the pores in the heat-receiving and heat-delivering or discharging surfaces depends not only on the quantity of the liquid charged into the tube and on the capillary action but also on the quantity of heat supplied. The dependence on these factors is particularly noted in capillary structures where the pore size distribution has a very narrow maximum range. To be able to operate at all times in the vicinity of the minimum AT, even at variable heat flow, it is important to obtain the pore size distribution shown in FIG. 2.

To demonstrate the improvements obtained with the capillary structure formed in accordance with the present invention, measurements have been taken on two heat pipes each formed of copper and each using water as the liquid working medium. In each of the heat pipes the heat flow density in the heating zone, that is, heatreceiving zone, was great and in the cooling zone or the heat-delivering zone was small, accordingly, it suffices to describe the effects in the heating zone.

In heat pipe 1 a 2 mm sintered layer of copper particles having a grain size range of 125 to 250 microns was formed. The pore size distribution curve was relatively pointed, that is it did not have a broad maximum size range. Just enough water was placed in the heat pipe so that all of the pores were full of water.

In heat tube 2, in'the heating zone, a 2 mm sintered layer was formed consisting of copper filings having an oblong shape with a length of about 1.5 mm and copper particles of spherical shape with a diameter of about 50 microns. Before the sintering operation, the coarse and fine particles were mixed in a ratio by weight of 2:1.

The quantity of water was varied until the temperature difference between a pore in the heat-receiving surface of the heat tube and the interior of the heat tube assumed a minimum at constant heat supply.

In FIG. 3 the temperature differences measured on heat tube 1 and heat tube 2 in the heat-receiving sur-v face are plotted as a function of the heat flow density at the heat-receiving surfaces. As can be seen in FIG.

, 3, the heat tube 2 greater heat flow densities were attained at equal temperature differences. As or perhaps more important in some instances, is the fact that at equal heating flux densities the temperature difference AT can be reduced. In FIG. 3 the range'between the upwardly extending dashed lines represents ebullient boiling while the space to the left of the left hand dashed line represents surface evaporation. It will be evident from FIG. 3, by changing over from heat tube 1 to heat tube 2 the passage from the range of ebullient boiling to surface evaporation is facilitated. In the region of the change-over from surface evaporation to ebullient boiling the reduction in AT is greater because in this region the curve is flatter.

What is claimed is:

1. A heat pipe comprising a closed tube having a heat-receiving surface and a heat-delivery surface, a layer of capillary structure sintered to the inside surface of said tube at least on the heat-receiving surface and the heat-delivery surface therein and another capillary structure interconnecting the heat-receiving and the heat-delivery surface for liquid transport therebetween, said tube and layer ofcapillary structure defining an enclosed space in the heat pipe for transporting vapor between the heat-receiving surface and the heat delivery surface, wherein the improvement comprises that the layer of capillary structure consists of pores of different sizes, and a vaporizable liquid filled into said tube in a quantity to fill only the smaller sized pores by capillary force, said pores are of a size in which capillary force takes effect and consists of a first group having a range of pores of relatively small sizes and a second group having a range of pores of relatively large sizes and within each of said first group and said second group there is a broad range of a maximum number of relatively small pore sizes and a broad range of a maximum number of relatively large pore sizes and the two the two ranges of maximum sizes, and the quantity of said working liquid being just sufficient to fill the relatively small sized pores in said first group because of the greater capillary force exerted by such pores.

2. A heat pipe, as set forth in claim 1, wherein said pores in said first group have approximately the same total volume as said pores in said second group.

3. A heat pipe comprising a closed tube having a heat-receiving surface and a heat-delivery surface, a layer of capillary structure sintered to the inside surface of said tube at least on the heat-receiving surface and the heat-delivery surface therein and another capillary structure interconnecting the heat-receiving and the heat-delivery surface for liquid transport therebetween, said tube and layer of capillary structure defining an enclosed space in the heat pipe for transporting vapor between the heat-receiving surface and the heatdelivery surface, wherein the improvement comprises that the layer of capillary structure consists of pores of different sizes, and a vaporizable liquid filled into said tube in a quantity to fill only the smaller sized pores by capillary force, said pores comprise a first group of pores and a second group of pores and the pores in said first group being distinctly smaller and of substantially the same size as compared to the pores in said second group which are of substantially the same size and both of said first and second groups having a maximum number of pores as compared to similarly sized other said pores in said capillary structure. 

1. A heat pipe comprising a closed tube having a heat-receiving surface and a heat-delivery surface, a layer of capillary structure sintered to the inside surface of said tube at least on the heat-receiving surface and the heat-delivery surface therein and another capillary structure interconnecting the heatreceiving and the heat-delivery surface for liquid transport therebetween, said tube and layer of capillary structure defining an enclosed space in the heat pipe for transporting vapor between the heat-receiving surface and the heat delivery surface, wherein the improvement comprises that the layer of capillary structure consists of pores of different sizes, and a vaporizable liquid filled into said tube in a quantity to fill only the smaller sized pores by capillary force, said pores are of a size in which capillary force takes effect and consists of a first group having a range of pores of relatively small sizes and a second group having a range of pores of relatively large sizes and within each of said first group and said second group there is a broad range of a maximum number of relatively small pore sizes and a broad range of a maximum number of relatively large pore sizes and the two maximum ranges are spaced apart by a range of pore sizes which fall between the pore sizes represented by the tWo ranges of maximum sizes, and the quantity of said working liquid being just sufficient to fill the relatively small sized pores in said first group because of the greater capillary force exerted by such pores.
 2. A heat pipe, as set forth in claim 1, wherein said pores in said first group have approximately the same total volume as said pores in said second group.
 3. A heat pipe comprising a closed tube having a heat-receiving surface and a heat-delivery surface, a layer of capillary structure sintered to the inside surface of said tube at least on the heat-receiving surface and the heat-delivery surface therein and another capillary structure interconnecting the heat-receiving and the heat-delivery surface for liquid transport therebetween, said tube and layer of capillary structure defining an enclosed space in the heat pipe for transporting vapor between the heat-receiving surface and the heat-delivery surface, wherein the improvement comprises that the layer of capillary structure consists of pores of different sizes, and a vaporizable liquid filled into said tube in a quantity to fill only the smaller sized pores by capillary force, said pores comprise a first group of pores and a second group of pores and the pores in said first group being distinctly smaller and of substantially the same size as compared to the pores in said second group which are of substantially the same size and both of said first and second groups having a maximum number of pores as compared to similarly sized other said pores in said capillary structure. 